Stimulation of mast cells and basophils upon contact of allergy-specific IgE antibodies with antigens, called immediate hypersensitivity, is one of the most powerful effector mechanisms of the mammalian immune system. Due to a combination of genetic predisposition and environmental stimuli, approximately 20% of the U.S. population is prone to develop an abnormally strong immediate hypersensitivity, a condition known as allergy. Physiological symptoms include increased vascular permeability, vasodilation, smooth muscle contraction, and local inflammation. These and other IgE dependent reactions can cause allergic diseases like allergic rhinitis (hay fever), asthma, atopic dermatitis (chronic skin irritations) and in the most severe cases can lead to anaphylactic shock, causing death of the individual by asphyxiation and cardiovascular collapse. Common environmental allergens are pollen, dust mites, certain foods, animal dander, fungal spores, and insect venoms.
The first exposure to a specific antigen can lead to the sensitization of the individual. The allergen binds with low specificity to pre-existing IgE in the plasma. This complex interacts with the low affinity receptor Fc.epsilon.RII on antigen presenting cells (APC). The antigen is internalized, proteolytically processed and transported to the surface of the APC by class II MHC molecules. Fragments of the antigen are thereby presented to CD4.sup.+ T helper cells which in turn activate IgE committed B cells to produce antigen-specific IgE. Normally IgE occurs in the human plasma at a concentration of about 0.2 mg/ml but in atopic patients this level can rise to a concentration of over 10 mg/ml.
Re-exposure to the allergen results in tight binding to the allergen-specific IgE present on the high-affinity receptor Fc.epsilon.RI on the surface of mast cells. Multivalent allergens cause the crosslinking of several receptors in the cell membrane. This triggers an intracellular signaling cascade, leading ultimately to the release of preformed mediators from cytoplasmic granules and the secretion of newly synthesized mediators. These mediators, notably histamines, leukotrienes, prostaglandins, and proteases, in turn cause the wide spectrum of symptoms of the allergic response. Furthermore, the release of chemotactic cytokines from the mast cell attracts and activates inflammatory cells to the location of antigen exposure. Finally, the release of IL-4 activates B cells to produce more antigen-specific IgE, thereby amplifying the allergic response. For a review, see Sutton et al. (1993) Nature 366: 421-428.
Mounting evidence indicates that the IgE system has evolved to cope primarily with infections by parasitic worms like Schistosoma mansoni. In the absence of such parasites, IgE mediated responses seem to be dispensable and frequently lead to pathologic consequences. Supporting this hypothesis is the fact that murine strains deficient in IgE or the IgE high-affinity receptor (Dombrowicz, et al. (1993) Cell 75: 969-976) lack the anaphylactic response, but appear otherwise normal.
IgE is a 190 kD antibody consisting of two .epsilon. heavy chains (70 kD) and two light chains (25 kD). The heavy chains contain one variable domain (V.sub.H) and four constant domains (C.sub.H 1 to C.sub.H 4). The light chains contain one variable domain (V.sub.L) and one constant domain (C.sub.L). Each of these immunoglobulin domains consists of about 100 residues and is stabilized by intramolecular sulfur bridges. The heavy and light chains are connected by intermolecular sulfur bridges.
The IgE molecule can be subdivided into the F.sub.AB (antigen binding) region, containing the variable and the first constant domains and the F.sub.C (crystalline) region, consisting of the remaining constant domains. The antigen binds to hypervariable sites within the variable region, whereas the IgE receptors bind to the F.sub.C region. The high-affinity IgE receptor Fc.epsilon.RI contacts a dodecapeptide sequence located at the N-terminus of the C.sub.H 3 domain and the low affinity IgE receptor Fc.epsilon.RII binds to the middle portion of the same domain (reviewed in Sutton, et al., supra).
The IgE molecule is significantly bent, reducing its predicted length from 17.5 nm for a planar molecule to 7 nm. This bend occludes one of the two potential Fc.epsilon.RI receptor binding sites resulting in a monovalent IgE molecule which, in the absence of a multivalent allergen, cannot crosslink receptor molecules to initiate the allergic response.
To allow the antigen mediated triggering of the allergic response, IgE must form a complex with the high affinity receptor, Fc.epsilon.RI. Fc.epsilon.RI consists of four transmembrane polypeptides: a, b, and g.sub.2. The a subunit, Fc.epsilon.RI(a), contains two extracellular immunoglobulin domains and it is the second domain, a(2), that binds to the convex site of the IgE molecule. The dissociation constant of this interaction is approximately 10.sup.-10 M (Sutton, et al., supra). The b and g chains of Fc.epsilon.RI are necessary to anchor the receptor in the cell membrane, to allow receptor crosslinking, and for signal transduction to initiate the release of mediators from mast cells.
To inhibit immediate hypersensitivity numerous steps of the pathway can be targeted. It should be possible to prevent the synthesis of IgE by binding to and blocking the action of IL-4 or the IL-4 receptor, to prevent mast cell and basophil stimulation by blocking IgE or the Fc.epsilon.RI IgE receptor, to prevent release of mediators by blocking a step of the intracellular signaling pathway, or to prevent physiological responses of patients by blocking the released mediators. This work demonstrates the use of high-affinity oligonucleotides to human IgE to inhibit the interaction of IgE with the Fc.epsilon.RI receptor.